It is applicable to optical telecommunications and more particularly to digital transmissions by means of optical fibers, especially those which make the most of the solitons and/or wavelength multiplexing.
At the current moment, only semiconductor lasers are able to obtain extremely short optical pulses while being compatible with transmissions by optical fibers (with these lasers, it is possible to obtain high repetition frequencies and embody compact pulse generating devices).
Among the known techniques for generating optical pulses, two prove to be advantageous, especially for the transmission of information via the soliton effect, namely the technique using a semiconductor laser which functions in the locking mode, and the technique using a semiconductor laser which functions in gain commutation.
In the case of the technique using a laser which functions in the locking mode, the Fabry-Perot type cavity of the laser is extended, that is its initial length Lo (about between 0.3 and 1 mm) is raised to L1 much higher than Lo (L1 about between 3 and 5 cm) so that the spacing Df of the lines of the Fabry-Perot cavity is located in the range of frequencies of between about 2 and 5 GHz.
This is obtained by making one of the faces of the semiconductor laser undergo an antireflection treatment and by placing a mirror, or better still a reflecting diffraction network, at the distance L1 from the other face of the semiconductor laser.
The locking of modes is embodied by modulating the injection current of the amplifier medium of the laser at the frequency Df.
By making the diffraction network rotate, this laser may be rendered wavelength-tunable.
This type of optical pulse generator has two major drawbacks:
the generator is relatively difficult to produce: the anti-reflection treatment needs to be of extremely good quality and the mechanical tolerances are extremely severe, or else this results in a lack Of sturdiness, PA1 the repetition frequency of the pulses is imposed by the length L1 of the extended cavity. PA1 Article by D. M. BIRD and al and entitlted "Miniature packaged actively mode-locked semiconductor laser with tunable 20 ps transform limited pulses" published in Electronic Letters, 6 Dec. 1990, vol 26, No 25, pp 2086-2087. PA1 Article by M. Cavelier, N. Stelmakh, J. M. Xie, L. Chusseau, J. M. Lourtiog, C. Kazmierski and N. Boudma and entitled "Picosecond (&lt;2.5 ps) Wavelength-Tunable (.sup..about. 20 nm/semiconductor laser pulses with repetition rates up to 12 GHz" published in Electronic Letters (1992), vol 28, No 3, pp 224-226. PA1 a semiconductor laser is made to function in gain commutation, PA1 a large portion (that is, at least one hundredth) of the light produced by this laser is reinjected into the laser, PA1 the light to be reinjected is previously filtered by an optical filter which is tuned to one of the modes of the laser, and PA1 one portion of the light produced by the laser is used as an optical signal after having had this portion been filtered by the optical filter. PA1 a semiconductor laser; PA1 means for controlling the semiconductor laser and able to have this laser function in gain commutation, and PA1 supply and reinjection means provided to reinject into the laser a large portion of the light produced by this laser and provide as an optical signal one portion of the light produced by this laser, these supply and reinjection means including an optical filter tuned to one of the modes of the laser and which firstly filters the light to be reinjected and the light forming the optical signal. PA1 a 2.times.2 type optical coupler having 4 accesses, one of said accesses receiving the light produced by the laser, this light thus reaching two other accesses, and PA1 an optical fiber loop which connects these two other accesses to each other by means of the optical filter, the optical signal being available to the fourth access. PA1 a 2.times.2 type optical coupler having four accesses with one receiving the light produced by the laser, this light thus reaching two of the other accesses, PA1 an optical isolator, and PA1 an optical fiber loop which connects these two other accesses to each other by means of the optical isolator and the optical filter, the isolator and filter being inserted in the loop so as to only allow the light circulating in this loop to pass, the optical signal being available to the fourth access. PA1 an optical coupler with at least three accesses, one access being optically connected to the front face of the semiconductor laser so as to receive the light coming from this front face and then arriving at the other two accesses, one of said accesses being optically connected to the rear face of the laser so as to reinject there a large portion of the light coming from the front face, whereas the other provides the optical signal, and PA1 an optical isolator, this isolator and the optical filter being inserted in the optical link between the front face of the laser and the optical coupler. PA1 an electric pulse generator, PA1 a polarization T which is constituted by a condensor and an inductive resistor and by means of which the laser connected to this generator, and PA1 an electric isolator which is placed between the generator and the polarization T and which differentiates the electric pulses produced by the generator.
However, it is possible to render this length L1 variable as described in the following document:
But the additional adjustment described in this article adds to the difficulty of embodying an optical pulse generator and increases the sensitivity of the latter to mechanical and external thermic stresses.
With regard to the known technique which uses a semiconductor laser functioning in gain commutation, this laser functions when the injection current of this laser is strongly and rapidly modulated with a clear passage of this injection current under the laser emission threshold.
Also, the relation between the modulation signal and the modulated optical power is distinctly non-linear which, under certain conditions, makes it possible to obtain extremely short optical pulses.
The advantages of gain commutation are its simplicity, the sturdiness and the possibility of continuously adjusting the repetition frequency over a wide range of frequencies.
The major drawback of this gain commutation technique resides in the presence of a high "chirp" (frequency fluctuation), that is a significant variation of the wavelength of the optical carrier wave during the pulse.
With a mono-mode laser (generally a DFB type laser), the "chirp" may be reduced by means of an optical treatment which generally makes use of a special optical fiber whose dispersion is negative, and/or an optical filter.
Another drawback of an optical pulse generator using gain commutation is that it is not tunable.
However, recently, a wavelength-tunable optical pulse generator using the principle of gain commutation has been described in the following document:
The Fabry-Perot type multi-mode semiconductor laser, which is used in the generator described in this document, is firstly rendered monomode, and secondly wavelength-tunable per pitch by means of a slight reinjection of the light filtered by a wavelength-tunable filter (the optical power reinjected into the laser is about one thousandth of the optical power produced by the latter or less).
However, the generator described in this article has a highly significant "chirp" which renders this generator unsuitable for transmissions by optical fibers causing the soliton phenomenon to intervene.